An impact tool being a configuration of a power tool generates a rotary impact force with a motor as a drive source to rotate a tip tool to intermittently give an impact force thereto to perform a work such as thread fastening, etc., and is presently used widely since the impact tool has a feature in that reaction is small, a clamping capacity is high, and so forth. Since such impact tool includes a rotary impact mechanism to generate a rotary impact force, however, noise while working is large to cause a problem.
FIG. 12 shows a longitudinal cross section of a general impact tool used conventionally.
The conventional impact tool shown in FIG. 12 comprises a cell pack 1 as an electric source, and a motor 2 as a drive source, and drives a rotary impact mechanism part to give rotation and impact to an anvil 3, thereby intermittently transmitting a rotary impact force to a tip tool 4 to perform a work such as screwing, etc.
In the rotary impact mechanism part built in a hammer casing 5, rotation of an output shaft (a motor shaft) of the motor 2 is reduced in speed through a planetary gear mechanism 6 to be transmitted to a spindle 7, so that the spindle 7 is rotationally driven at a predetermined speed. Here, the spindle 7 and a hammer 8 are connected to each other by a cam mechanism, the cam mechanism comprising a V-shaped spindle cam groove 7a formed on an outer peripheral surface of the spindle 7, a V-shaped hammer cam groove 8a formed on an inner peripheral surface of the hammer 8, and balls 9 that engage with the cam grooves 7a, 8a. Also, the hammer 8 is constantly biased toward a tip end (rightward in FIG. 12) by a spring 10, and positioned with a clearance from an end surface of the anvil 3 by means of engagement of the balls 9 and the cam grooves 7a, 8a when being stationary. Projections, respectively, are formed symmetrically in two locations on opposite rotary flat surfaces of the hammer 8 and the anvil 3. In addition, a screw 11, the tip tool 4, and the anvil 3 are constrained relative to one another in a direction of rotation. Also, in FIG. 12, the reference numeral 14 denotes a bearing metal that bears the anvil 3 rotatably.
As described above, when the spindle 7 is rotationally driven, rotation thereof is transmitted to the hammer 8 through the cam mechanism, and the projection on the hammer 8 engages with the projection on the anvil 3 to rotate the anvil 3 before the hammer 8 makes a half revolution, but when relative rotations are generated between the hammer 8 and the spindle 7 by reaction forces of the engagement, the hammer 8 begins to retreat toward the motor 2 while compressing the spring 10 along a spindle cam groove 7a. When backward movement of the hammer 8 causes the projection on the hammer 8 to get over the projection on the anvil 3 to release engagement of the both, the hammer 8 is quickly accelerated in a direction of rotation and forward owing to elastic energy accumulated in the spring 10 and the action of the cam mechanism in addition to torque of the spindle 7 to be moved forward by the bias of the spring 10, and the projection thereon engages again with the projection on the anvil 3 to begin to rotate together. At this time, since a large rotary impact force is applied to the anvil 3, the rotary impact force is transmitted to the screw 11 through the tip tool 4 mounted to the anvil 3.
Thereafter, the same actions are repeated, the rotary impact force is intermittently and repeatedly transmitted to the screw 11, and the screw 11 is screwed into a timber 12 being a clamped object.
By the way, since the hammer 8 makes longitudinal movements simultaneously with rotary movements in a work, in which such impact tool is used, these movements serve as a source of vibration to axially vibrate the timber 12, being a clamped object, through the anvil 3, the tip tool 4, and the screw 11 to generate a large noise.
Here, it is found that a noise energy from an object being clamped accounts for a large ratio in noise while working with the use of an impact tool, it is required that a vibration force transmitted to an object being clamped be restricted to a small extent in order to achieve reduction in noise, and various measures have been examined (see, for example, JP-A-7-237152 and JP-A-2002-254335).